add startcanvas/endcanvas entry-points for the script. rename all "official" entry-points to use "sk_scrape_" prefix

BUG=

Review URL: https://codereview.chromium.org/15511006

git-svn-id: http://skia.googlecode.com/svn/trunk@9216 2bbb7eff-a529-9590-31e7-b0007b416f81
2 files changed